An arrhythmia is an abnormal heart rhythm. One example of an arrhythmia is bradycardia wherein the heart beats at an abnormally slow rate or wherein significant pauses occur between consecutive beats. Other examples of arrhythmias include tachyarrhythmias wherein the heart beats at an abnormally fast rate. With atrial tachycardia, the atria of the heart beat abnormally fast. With ventricular tachycardia, the ventricles of the heart beat abnormally fast. Though often unpleasant for the patient, an atrial tachycardia is typically not fatal. However, atrial fibrillation can cause stroke and other symptoms. Also, ventricular tachycardia can trigger ventricular fibrillation wherein the heart beats chaotically resulting in little or no net flow of blood from the heart to the brain and other organs. Ventricular tachycardia and ventricular fibrillation, if not terminated, are fatal. Hence, it is highly desirable to prevent or terminate arrhythmias, particularly ventricular tachycardias.
One technique for preventing arrhythmias is to overdrive pace the heart wherein an implantable cardiac stimulation device, such as a pacemaker or implantable cardioverter defibrillator (ICD), applies electrical pacing pulses to the heart at a rate somewhat faster than the intrinsic heart rate of the patient having a normal sinus rhythm. For bradycardia, the cardiac stimulation device may be programmed to pace the heart at a rate of 60 to 80 pulses per minute (ppm) to thereby prevent the heart from beating too slow and to eliminate any long pauses between heartbeats. To prevent tachyarrhythmias from occurring, the cardiac stimulation device paces the heart at a rate slightly faster than the intrinsic heart rate of the patient. In other words, overdrive pacing is applied and maintained in an effort to prevent an actual tachycardia from arising.
One particular technique is described in U.S. patent application Ser. No. 09/471,788 entitled “Improved Method for Establishing Pacing Rate During Dynamic Atrial Overdrive Pacing” by Joe Florio et al. filed Dec. 23, 1999. In that technique, the stimulation device monitors the heart of the patient and, if two consecutive intrinsic heartbeats are detected (or two intrinsic beats are detected within a predetermined number of cycles such as within the last sixteen cycles), overdrive pacing is automatically triggered. The overdrive pacing rate is calculated based on the heart rate detected at the time overdrive pacing is triggered and is typically 5 to 10 ppm higher than the intrinsic rate, i.e. an overdrive pacing rate increment of 5 to 10 bpm is added to the detected intrinsic rate. The intrinsic heart rate may be determined, for example, by calculating the time interval between the two consecutive intrinsic beats. The stimulation device then overdrive paces the heart at the selected overdrive pacing rate for a programmed number of overdrive events or “overdrive cycles”. Thereafter, the stimulation device slowly decreases the overdrive pacing rate by a rate decrement specified by a programmed “recovery rate” until additional intrinsic beats are detected, then the device repeats the process to determine a new overdrive pacing rate and paces accordingly. If the heart rate is increasing quickly, such as may occur with an episode of tachycardia, the stimulation device may still detect intrinsic beats even while overdrive pacing is being applied. If so, the stimulation device immediately calculates a new higher overdrive pacing rate. If intrinsic beats are still detected, the overdrive pacing rate is increased again up to a maximum overdrive limit such as a preprogrammed maximum sensor rate.
Ultimately, the overdrive pacing rate will be increased to the point where it exceeds the intrinsic rate and hence no intrinsic beats will be detected. The pacing rate is eventually decreased using a programmed recovery rate until two or more consecutive intrinsic beats are again detected and the pacing rate is increased again. Assuming that overdrive pacing has succeeded in preventing tachycardia, the recovery rate will ensure that the pacing rate decreases slowly back down to a programmed base rate or sensor indicated rate. For example, if the base rate is programmed at 60 bpm, the heart will be paced at the base rate even if the recovery rate would otherwise cause the rate to decrease even further. Likewise, if an alternative base rate, such as the rest rate or circadian base rate is programmed, the pacing rate will not fall below those rates either.
It is believed that overdrive pacing is effective for at least some patients for preventing the onset of an actual tachycardia for the following reasons. A normal, healthy heart beats only in response to electrical pulses generated from a portion of the heart referred to as the sinus node. The sinus node pulses are conducted to the various atria and ventricles of the heart via certain, normal conduction pathways. In some patients, however, additional portions of the heart also generate electrical pulses referred to as “ectopic” pulses. Each pulse, whether a sinus node pulse or an ectopic pulse has a refractory period subsequent thereto during which time the heart tissue is not responsive to any electrical pulses. A combination of sinus pulses and ectopic pulses can result in a dispersion of the refractory periods, which, in turn, can trigger a tachycardia. By overdrive pacing the heart at a uniform rate, the likelihood of the occurrence of ectopic pulses is reduced and the refractory periods within the heart tissue are rendered uniform and periodic. Thus, the dispersion of refractory periods is reduced and tachycardias triggered thereby are substantially avoided.
Thus it is desirable within patients prone to tachyarrhythmias to ensure that most beats of the heart are paced beats, as any unpaced beats may be ectopic beats. A high percentage of paced beats can be achieved simply by establishing a high overdrive pacing rate. However, a high overdrive pacing rate has disadvantages as well. For example, a high overdrive pacing rate may be unpleasant to the patient, particularly if the artificially-induced heart rate is relatively high in comparison with the heart rate that would otherwise normally occur. A high heart rate may also cause possible damage to the heart or may possibly trigger more serious arrhythmias, such as a ventricular fibrillation. A high overdrive pacing rate may be especially problematic in patients suffering from heart failure, particularly if the heart failure is due to an impaired diastolic function. A high overdrive pacing rate may actually exacerbate heart failure in these patients. Also, a high overdrive pacing rate may be a problem in patients with coronary artery disease because increasing the heart rate decreases diastolic time and decreases perfusion, thus intensifying ischemia. Also, the need to apply overdrive pacing pulses operates to deplete a power supply of the implantable cardiac stimulation device, perhaps requiring frequent surgical replacement of the device.
The various overdrive pacing parameters (the overdrive pacing rate increment, the number of overdrive cycles, the recovery rate decrement) are programmed by a physician using an external programmer in an attempt to ensure that an optimal degree of overdrive pacing is achieved of typically 85% to 95%. Unfortunately, it is quite difficult for a physician to initially determine the parameters needed to achieve the desired degree of overdrive pacing within a particular patient. Instead, the physician typically sets the various control parameters of the stimulation device of the patient to default values and then programs the device to record the resulting degree of overdrive pacing as a function of heart rate. The patient is sent home and, weeks or months later, the patient returns to the physician for a follow-up session to permit the physician to review the recorded information and to determine whether the default parameters achieved the desired degree of overdrive pacing. If the degree of overdrive pacing is too low, perhaps only 50%, the physician typically increases the number of overdrive cycles or selects a more aggressive overdrive pacing rate increment. If the degree of overdrive pacing is too high, perhaps 100%, the physician may decrease the number of overdrive cycles or may select a lower overdrive pacing rate increment. The patient is again sent home and, weeks or months later, the patient again returns to the physician so that the physician can again review the recorded degree of overdrive pacing and, if needed, re-set the number of overdrive events or the overdrive pacing rate. This process is usually repeated several times over a period of months until a number of overdrive events and an overdrive pacing rate increment are identified that comes closest to achieving the desired degree of overdrive pacing. During this process the physician may also adjust any of the other parameters as well, such as the base rate, recovery rate etc.
Although the aforementioned technique is generally effective to control overdrive pacing, room for improvement remains. One particular area of concerns relates to the difficulty in programming an optimal number of overdrive events or cycles. Typically, the number of overdrive events is set to a value within a range 10–90 cycles. While the intrinsic heart rate of the patient is decreasing, a small number of cycles is preferred as that ensures that the overdrive pacing rate promptly decreases to the intrinsic rate to prevent unnecessary and prolonged overdrive pacing at a rate much higher than the intrinsic rate. However, while the intrinsic rate of the patient is generally stable or slowly increasing, a large number of cycles is preferred as that prevents frequent changes in the overdrive pacing rate, which can be an annoyance to the patient. As can be appreciated, if the number of overdrive cycles must be set by the physician to a single value that remains fixed until a subsequent follow-up session, optimal overdrive pacing may not always be achieved.
FIGS. 1 and 2 illustrate these problems. FIG. 1 shows an intrinsic rate 2 and a resulting overdrive pacing rate 3 wherein the intrinsic rate varies fairly quickly and the number of overdrive pacing cycles 4 is set fairly high. As can be seen, the overdrive pacing rate reacts fairly slowly to changes in the intrinsic heart rate, resulting in an overdrive pacing rate that is considerably higher than necessary at times. FIG. 2 shows an intrinsic rate 6 and a resulting overdrive pacing rate 7 wherein the intrinsic rate is fairly stable and the number of overdrive pacing cycles 8 is set fairly low. As can be seen, the overdrive pacing rate varies considerably even though the intrinsic rate is stable; resulting in frequent unwanted changes in heart rate.
Accordingly, it would be desirable to provide an improved overdrive pacing technique, which does not employ a fixed number of overdrive cycles, and it is to that end that the invention is primarily directed.